The invention relates to a method for flexographic multi-color printing.
In a typical flexographic multi-color print process, an endless web of a print medium, e.g. paper, is passed around a major portion of the periphery of a central impression cylinder, and, while the central impression cylinder is rotating, inks of different colors are successively printed onto the web by means of ink units that are distributed along the periphery of the central impression cylinder. Each ink unit comprises a print cylinder, which is adjusted against the web on the periphery of the central impression cylinder and carries a printing plate which defines a print pattern that corresponds to a color component of the image to be printed. The ink is applied to the printing cylinder by means of an anilox roller the peripheral surface of which has a regular pattern of finely distributed pits. The periphery of the anilox roller passes through an ink fountain, e.g. a chambered doctor blade, where the pits are filled with ink. When the anilox roller rotates, the ink to the pits is transferred to a nip formed between the anilox roller and the printing cylinder, and, in this nip, the ink is transferred onto elevated, printing parts of the print pattern that is formed on the printing plate.
The color image that is to be printed on the endless web has a certain repeat length which defines the required peripheral length of the printing cylinders. Since the color component images that are printed with the printing cylinders of all the ink units contribute to the same image, they all have the same repeat length, and, accordingly, the printing cylinders of all ink units have the same diameter. Likewise, in a conventional flexographic printing press, the anilox rollers of all ink units have the same diameter.
The peripheral speed of the anilox roller is equal to the peripheral speed of the printing cylinder which itself is essentially equal to the speed with which the web is advanced on the peripheral surface of the central impression cylinder, i.e. the printing speed. Due to the rotation of the anilox roller, the ink in the pits at the peripheral surface of the anilox roller are subject to centrifugal forces which have to be overcome by adhesive forces between the liquid ink and the surface of the anilox roller. Since the centrifugal forces increase with increasing speed of rotation of the anilox roller, the printing speed can only be increased up to a limit, where the centrifugal forces would overcome the adhesive forces between the ink and the anilox roller. Since, for a given printing speed and hence a given peripheral speed of the anilox roller, the centrifugal forces are inversely proportional to the diameter of the anilox roller, this diameter should be increased in order to permit a higher printing speed. However, an increased diameter of the anilox rollers leads not only to increased manufacturing costs for the printing press, increased power consumption due to increased weight and moment of inertia of the anilox rollers, and to increased overall dimensions of the printing machine, but also implies longer transfer times for the ink on its way from the ink fountain to the nip between the anilox roller and the printing cylinder. When rapidly drying inks are employed, in order to permit high printing speeds, the increased transfer time may give rise to problems in an initial set-up phase of a print run, in which the machine is operated with a reduced speed. Then, the transfer time may become so long that, undesirably, a portion of the ink dries-out already on the surface of the anilox roller. Thus, a suitable compromise has to be made for the diameter of the anilox rollers.